One example of conventional vapor compression refrigeration systems provided with a plurality of compression mechanisms are air conditioning systems used to air-condition buildings. This kind of air conditioning system is provided with a plurality of user units and a heat source unit capable of accommodating the heating and cooling loads of the user units. In order to enable the system to be operated in a partial load mode, the heat source unit is provided with a circuit configuration made up of a plurality of comparatively small-capacity compression mechanisms connected in parallel. The compression mechanisms are provided with an oil equalizing circuit including oil separators connected to the discharge sides of the compression mechanisms, oil return pipes for returning the oil separated by the oil separators to the compression mechanisms, and oil equalizing pipes connected between the compression mechanisms for reducing imbalances in the amount of oil in the compression mechanisms.
In the conventional oil equalizing circuit just described, the amount of oil in each compression mechanism is ensured by providing oil return pipes and oil equalizing pipes. However, there are times when air conditioning systems provided with a plurality of compression mechanisms are run in a partial load mode in which some compression mechanisms are running and some are stopped and times when such systems are run in a partial load mode in which some compression mechanisms are operated with a reduced operating load using inverter control or the like. During these various operating patterns, it is sometimes difficult to supply oil sufficiently to the compression mechanisms that are running. Thus, the reliability of the oil supply achieved with conventional oil equalizing circuits is insufficient.